Muscimol, a GABA receptor agonist, and diazepam which indirectly activates GABA receptors decreased the turnover rate of acetylcholine in midbrain and cortex but failed to change the turnover rate in striatum or hippocampus. These results suggest that GABA receptors participate in the modulation of acetylcholine metabolism in some areas but not in others. The similar profile of muscimol and diazepam in the regulation of ACh turnover further suggest the view that diazepam may act by facilitating GABA-ergic transmission in brain.